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package java.nio.channels.spi;

import java.io.IOException;
import java.net.ProtocolFamily;
import java.nio.channels.*;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.Iterator;
import java.util.ServiceLoader;
import java.util.ServiceConfigurationError;
import sun.security.action.GetPropertyAction;


/**
 * Service-provider class for selectors and selectable channels.
 *
 * <p> A selector provider is a concrete subclass of this class that has a
 * zero-argument constructor and implements the abstract methods specified
 * below.  A given invocation of the Java virtual machine maintains a single
 * system-wide default provider instance, which is returned by the {@link
 * #provider() provider} method.  The first invocation of that method will locate
 * the default provider as specified below.
 *
 * <p> The system-wide default provider is used by the static <tt>open</tt>
 * methods of the {@link java.nio.channels.DatagramChannel#open
 * DatagramChannel}, {@link java.nio.channels.Pipe#open Pipe}, {@link
 * java.nio.channels.Selector#open Selector}, {@link
 * java.nio.channels.ServerSocketChannel#open ServerSocketChannel}, and {@link
 * java.nio.channels.SocketChannel#open SocketChannel} classes.  It is also
 * used by the {@link java.lang.System#inheritedChannel System.inheritedChannel()}
 * method. A program may make use of a provider other than the default provider
 * by instantiating that provider and then directly invoking the <tt>open</tt>
 * methods defined in this class.
 *
 * <p> All of the methods in this class are safe for use by multiple concurrent
 * threads.  </p>
 *
 * @author Mark Reinhold
 * @author JSR-51 Expert Group
 * @since 1.4
 */

public abstract class SelectorProvider {

  private static final Object lock = new Object();
  private static SelectorProvider provider = null;

  /**
   * Initializes a new instance of this class.
   *
   * @throws SecurityException If a security manager has been installed and it denies {@link
   * RuntimePermission}<tt>("selectorProvider")</tt>
   */
  protected SelectorProvider() {
    SecurityManager sm = System.getSecurityManager();
    if (sm != null) {
      sm.checkPermission(new RuntimePermission("selectorProvider"));
    }
  }

  private static boolean loadProviderFromProperty() {
    String cn = System.getProperty("java.nio.channels.spi.SelectorProvider");
    if (cn == null) {
      return false;
    }
    try {
      Class<?> c = Class.forName(cn, true,
          ClassLoader.getSystemClassLoader());
      provider = (SelectorProvider) c.newInstance();
      return true;
    } catch (ClassNotFoundException x) {
      throw new ServiceConfigurationError(null, x);
    } catch (IllegalAccessException x) {
      throw new ServiceConfigurationError(null, x);
    } catch (InstantiationException x) {
      throw new ServiceConfigurationError(null, x);
    } catch (SecurityException x) {
      throw new ServiceConfigurationError(null, x);
    }
  }

  private static boolean loadProviderAsService() {

    ServiceLoader<SelectorProvider> sl =
        ServiceLoader.load(SelectorProvider.class,
            ClassLoader.getSystemClassLoader());
    Iterator<SelectorProvider> i = sl.iterator();
    for (; ; ) {
      try {
        if (!i.hasNext()) {
          return false;
        }
        provider = i.next();
        return true;
      } catch (ServiceConfigurationError sce) {
        if (sce.getCause() instanceof SecurityException) {
          // Ignore the security exception, try the next provider
          continue;
        }
        throw sce;
      }
    }
  }

  /**
   * Returns the system-wide default selector provider for this invocation of
   * the Java virtual machine.
   *
   * <p> The first invocation of this method locates the default provider
   * object as follows: </p>
   *
   * <ol>
   *
   * <li><p> If the system property
   * <tt>java.nio.channels.spi.SelectorProvider</tt> is defined then it is
   * taken to be the fully-qualified name of a concrete provider class.
   * The class is loaded and instantiated; if this process fails then an
   * unspecified error is thrown.  </p></li>
   *
   * <li><p> If a provider class has been installed in a jar file that is
   * visible to the system class loader, and that jar file contains a
   * provider-configuration file named
   * <tt>java.nio.channels.spi.SelectorProvider</tt> in the resource
   * directory <tt>META-INF/services</tt>, then the first class name
   * specified in that file is taken.  The class is loaded and
   * instantiated; if this process fails then an unspecified error is
   * thrown.  </p></li>
   *
   * <li><p> Finally, if no provider has been specified by any of the above
   * means then the system-default provider class is instantiated and the
   * result is returned.  </p></li>
   *
   * </ol>
   *
   * <p> Subsequent invocations of this method return the provider that was
   * returned by the first invocation.  </p>
   *
   * @return The system-wide default selector provider
   */
  public static SelectorProvider provider() {
    synchronized (lock) {
      if (provider != null) {
        return provider;
      }
      return AccessController.doPrivileged(
          new PrivilegedAction<SelectorProvider>() {
            public SelectorProvider run() {
              if (loadProviderFromProperty()) {
                return provider;
              }
              if (loadProviderAsService()) {
                return provider;
              }
              provider = sun.nio.ch.DefaultSelectorProvider.create();
              return provider;
            }
          });
    }
  }

  /**
   * Opens a datagram channel.
   *
   * @return The new channel
   * @throws IOException If an I/O error occurs
   */
  public abstract DatagramChannel openDatagramChannel()
      throws IOException;

  /**
   * Opens a datagram channel.
   *
   * @param family The protocol family
   * @return A new datagram channel
   * @throws UnsupportedOperationException If the specified protocol family is not supported
   * @throws IOException If an I/O error occurs
   * @since 1.7
   */
  public abstract DatagramChannel openDatagramChannel(ProtocolFamily family)
      throws IOException;

  /**
   * Opens a pipe.
   *
   * @return The new pipe
   * @throws IOException If an I/O error occurs
   */
  public abstract Pipe openPipe()
      throws IOException;

  /**
   * Opens a selector.
   *
   * @return The new selector
   * @throws IOException If an I/O error occurs
   */
  public abstract AbstractSelector openSelector()
      throws IOException;

  /**
   * Opens a server-socket channel.
   *
   * @return The new channel
   * @throws IOException If an I/O error occurs
   */
  public abstract ServerSocketChannel openServerSocketChannel()
      throws IOException;

  /**
   * Opens a socket channel.
   *
   * @return The new channel
   * @throws IOException If an I/O error occurs
   */
  public abstract SocketChannel openSocketChannel()
      throws IOException;

  /**
   * Returns the channel inherited from the entity that created this
   * Java virtual machine.
   *
   * <p> On many operating systems a process, such as a Java virtual
   * machine, can be started in a manner that allows the process to
   * inherit a channel from the entity that created the process. The
   * manner in which this is done is system dependent, as are the
   * possible entities to which the channel may be connected. For example,
   * on UNIX systems, the Internet services daemon (<i>inetd</i>) is used to
   * start programs to service requests when a request arrives on an
   * associated network port. In this example, the process that is started,
   * inherits a channel representing a network socket.
   *
   * <p> In cases where the inherited channel represents a network socket
   * then the {@link java.nio.channels.Channel Channel} type returned
   * by this method is determined as follows:
   *
   * <ul>
   *
   * <li><p> If the inherited channel represents a stream-oriented connected
   * socket then a {@link java.nio.channels.SocketChannel SocketChannel} is
   * returned. The socket channel is, at least initially, in blocking
   * mode, bound to a socket address, and connected to a peer.
   * </p></li>
   *
   * <li><p> If the inherited channel represents a stream-oriented listening
   * socket then a {@link java.nio.channels.ServerSocketChannel
   * ServerSocketChannel} is returned. The server-socket channel is, at
   * least initially, in blocking mode, and bound to a socket address.
   * </p></li>
   *
   * <li><p> If the inherited channel is a datagram-oriented socket
   * then a {@link java.nio.channels.DatagramChannel DatagramChannel} is
   * returned. The datagram channel is, at least initially, in blocking
   * mode, and bound to a socket address.
   * </p></li>
   *
   * </ul>
   *
   * <p> In addition to the network-oriented channels described, this method
   * may return other kinds of channels in the future.
   *
   * <p> The first invocation of this method creates the channel that is
   * returned. Subsequent invocations of this method return the same
   * channel. </p>
   *
   * @return The inherited channel, if any, otherwise <tt>null</tt>.
   * @throws IOException If an I/O error occurs
   * @throws SecurityException If a security manager has been installed and it denies {@link
   * RuntimePermission}<tt>("inheritedChannel")</tt>
   * @since 1.5
   */
  public Channel inheritedChannel() throws IOException {
    return null;
  }

}
